"Finally it became so ridiculous that Pope Gregory XIII was convinced by his astronomers that basically all the Christian holidays were being celebrated on the wrong days," Duncan said.

The pope introduced his Gregorian calendar in 1582, which determined that only one out of every four "century years" would observe a leap year. Thus while the years 2000 and 2400 are leap years, 2100, 2200, and 2300 are not.

The Gregorian calendar was gradually, and sometimes grudgingly, adopted by much of the world and remains in common use.

China's Solution

In China the Gregorian calendar is commonly used, but the traditional lunar-solar calendar is still observed to determine the dates of festivals like Chinese New Year.

As with other ancient calendars, the Chinese traditional calendar corresponds to the phases of the moon.

But the Chinese system includes a corresponding solar calendar and introduces an entire leap month about every third year to keep the calendar in synch with the seasons.

Chinese leap years of 13 months have 383, 384, or 385 days.

Such a system preserves a monthly cycle that begins with the new moon and centers on lunar events that are important for the timing of religious and cultural milestones.

Mayas' Missing Leap Year

The ancient Maya, famed for their elaborate and accurate calendar systems, observed two calendar years, but neither seemed to have bothered with a leap year.

"As far as we know, the people of Mesoamerica, the Maya included, didn't care about leap years," said Anthony Aveni, an expert in ancient Mesoamerican astronomy at Colgate University.

The Maya solar year of 365 days was central to the agricultural cycle, while their ritual year of 260 days was critical for determining auspicious dates.

These calendars were carefully designed to synchronize in 52-year cycles, but no effort was made to prevent "drifting" dates.

"They didn't care if they didn't have a white Christmas, or if their Fourth of July wasn't in the summer, to put it in our terms," Aveni explained.

The Maya instead placed priority on marking the passage of time through additional calendar systems such as the Long Count, which unfolds on a cycle more than 5,000 years long.

"Our philosophy about leap year is a complicated scheme to make the seasons jibe with the calendar," Aveni said.

"They were more concerned that time should be unbroken, not interfered with, and that the count of time should have continuity," he said.